In the past, broadband coaxial cable television systems have been designed with a system architecture known as "trunk and feeder". The function of a trunk coaxial cable is to deliver broadband television signals from a reception center, or headend, over the shortest distance with the least amount of amplification to a plurality of distribution points. The distribution points are connected to feeder coaxial cables which emanate from the trunk coaxial cable and contain subscriber tap off devices. At the distribution points, the feeders connect to the trunk at locations commonly termed trunk/bridger stations. Each trunk/bridger station has previously included a trunk amplifier for maintaining sufficient signal level through the trunk coaxial cable and a bridger amplifier for tapping off a portion of the trunk signal and distributing it to the feeders emanating from the trunk/bridger station. Because they are at the same location, the trunk amplifier and bridger amplifier have generally been contained within the same environmental housing. In coaxial trunk and feeder systems, the bridger locations (along with the headend and any hubs) are generally known as "star" focal points with the feeder cables emanating in all directions from them.
The locations of trunk/bridger stations along the trunk cable have been determined by the loss structure of the coaxial cable and the type of amplifier technology utilized. In general, the loss structure of the cable is determined by the channel loading of the particular cable television system. Coaxial cable in general appears as a distributed shunt capacitance along the length of a cable. Therefore, its loss characteristics vary with the length of the cable and the frequency of the signal. The longer the cable, the greater the attenuation, and the higher frequency, the greater the attenuation. With increases in channels more capacity has to be added at higher frequencies, and thus shorter distances between trunk amplifiers are necessary.
Many previous CATV systems in the industry were designed with a channel capacity of about 30-40 channels. Recently with the increase in available programming services, it has become desirable to upgrade these existing cable television systems to a much greater channel capacity, for example 60-80 channels. With these changes the loss structure of the cable varies from the original design which previously determined the locations of the trunk amplifier stations. A typical upgrade, from 30-35 channels (highest frequency 270-300 MHz) to 78 channels (highest frequency 550 MHz) requires that the trunk/bridger stations be relocated to compensate for the change in the loss structure of the system.
With regard to other types of reconfigurations, fiber optics have proven their ability to provide CATV systems with increased reliability and picture quality. Optical fibers have intrinsically more information carrying capacity than do the coaxial cables which are used in present CATV systems. In addition, optical fibers are subject to less signal attenuation per unit length than are coaxial cables adapted for carrying radio frequency signals. Consequently, optical fibers are capable of spanning longer distances between signal regenerators or amplifiers than are coaxial cable. In addition, the dielectric nature of optical fiber eliminates the possibility of signal outages caused by electrical shorting or radio frequency pickup. Finally, optical fiber is immune to ambient electromagnetic interference (EMI) and generates no EMI of its own.
There are numerous architectures in which optical fiber capability has been proven. These include the use of optical fiber as a fiber optic backbone and cable area networks. These CATV system builds have proved that such optical links are viable for CATV systems. However, both of these concepts are add-on layers developed for upgrading existing coaxial plants. But, while these optical fiber links have improved reliability and picture quality to the individual subscribers, they have also actually increased the electronics for the systems.
Recently a concept has evolved that, if an optical fiber could entirely replace a coaxial trunk system, then an entire layer of electronics could be eliminated thereby truly increasing reliability and lowering the cost associated with a major rebuild program. This concept is essentially what is termed "fiber to the feeder" (FTF) and in its simplest form envisions the replacement of the coaxial trunk system with optical fiber to what was a bridger location. The major problem with a fiber to feeder system is economics.
In answer to the high cost of fiber to the feeder systems, a new concept of "fiber to the service area" (FTSA) is now being implemented. Instead of running a trunk optical fiber to each distribution point, a number of distribution points in an immediate geographic area are grouped into a service area. The service area, rather than being geographically coextensive, could be to "pockets" of subscribers which require a specialized service, or for other reasons. The service area is then fed by an optical fiber trunk. This allows a rebuild of a system which increases channel capacity while using a new fiber optic backbone and not increasing cost.
In all present reconfiguration or rebuilding programs, whether merely for channel capacity upgrades, fiber to the feeder service, or for fiber to the service area, the distribution stars or bridger locations have to be moved. This is enormously expensive and it would be extremely advantageous to provide a system in which existing bridger locations could be maintained while taking advantage of these new technologies.